1. Field of the Invention
The present invention relates generally to the field of packaging materials, particularly packaging materials for use with moisture vapor and oxygen sensitive products. It concerns improving moisture vapor and oxygen barrier properties of polyethylene resins by blending them with relatively small amounts of low molecular weight hydrogenated aliphatic hydrocarbon resins.
2. Description of Related Art
Polyethylenes can be classified into two types: (i) high pressure branched low density polyethylene or LDPE and (ii) low pressure or linear polyethylene. Low pressure or linear polyethylene can be further separated into three categories: (iia) high density polyethylene or HDPE, (iib) linear low density polyethylene or LLDPE and (iic) ultra linear low density polyethylene or ULLDPE. Branched LDPE was the first type of polyethylene to be commercialized, and it can be polymerized in high pressure tubular or autoclave reactors. The density of branched LDPE is typically between about 0.910–0.940 g/cm3; it can be heavily and randomly branched with a significant fraction of its branches being longer than the critical entanglement molecular weight of polyethylene.
Polyethylenes with densities higher than about 0.940 g/cm3 are typically referred to as HDPE. These are produced with no comonomer or with very small amounts of comonomers such as α-olefins like hexene, butene, and octane, among others. Comonomer is mixed in with ethylene at low pressures during the polymerization process. HDPE can be produced in slurry loop reactors, solution form reactors or in gas phase fluidized bed reactors. Certain types of HDPE polymer have a small amount of long chain branching.
Linear LDPEs (LLDPEs) are obtained by incorporating sufficient α-olefin comonomers into linear polyethylene to produce polyethylene with a density between about 0.910–0.940 g/cm3. The α-olefin comonomers are essentially excluded from the crystal lattice of polyethylene; therefore, their presence serves to disrupt the crystallizability of the linear chain, which can lead to polyethylenes having lower densities (crystallinity). By definition, LLDPEs comprise essentially no (e.g., less than about 0.1% by weight of the polyethylene) long branches (e.g., branches longer than the critical entanglement molecular weight of polyethylene). These polymers can be produced in low pressure gas phase fluidized bed reactors, solution process reactors or in slurry loop reactors.
Ultra linear low density polyethylenes, ULLDPEs, which incorporate even higher levels of α-olefin comonomers into linear polyethylene have densities lower than about 0.910 g/cm3 and can be produced in reactors similar to those used to produce LLDPEs.
Polyethylenes can be used in the manufacture of a number of different packaging items using a variety of conversion processes such as blow molding, injection molding, sheet extrusion, blown film extrusion and cast film extrusion. Polyethylenes can also be used as components in multilayer packaging articles, which are manufactured using coextruded blown film, coextruded cast film, coextruded blow molding and other processes such as extrusion coating and laminations. Through the use of these various processing techniques, polyethylenes can be used to package a number of different food and non-food finished goods such as milk (e.g., blow molded bottles), bread (e.g., blown and cast films), paper products (e.g., extrusion coating, cast and blown film), applesauce (thermoformed sheet) and cleaning supplies (e.g., injection and blow molded bottles).
Polyethylenes are used in packaging items because they provide at least some resistance to moisture, gases, acids, bases, and solvents, while retaining package integrity by virtue of their impact strength, tear strength, stiffness, and other key attributes or properties. While certain polyethylenes have relatively low moisture vapor and oxygen transmission rates, there are other polymers with still lower moisture and oxygen transmission rates than polyethylenes. Thus, it would be advantageous to produce packaging items comprising polyethylene (e.g., HDPE, LLDPE, ULLDPE and LDPE) with improved moisture and oxygen barrier, while minimizing any increase in manufacturing cost or significantly decreasing other desirable properties imparted by polyethylene to the packaging item.